Until recently, chlorine was used as a bleaching agent in the production of white pulp and paper. However, environmental protection agencies have prohibited the use of chlorine for pulp bleaching since it has been found to produce highly hazardous organochlorine compounds. Chlorine dioxide has replaced chlorine as the primary bleaching agent in the production of white pulp and paper. Chlorine dioxide is also used for water purification. It is now the most important worldwide commodity for these purposes.
Existing technologies for commercial production of chlorine dioxide involves the reduction of sodium chlorate/chloric acid with reducing agents such as hydrogen peroxide and glycerol, glycol (See e.g. U.S. Pat. Nos. 5,093,097; 5,091,166; 5,380,517; 5,486,344; and 5,487,881 and Japanese Pat. No. JP88-8203), methanol (U.S. Pat. Nos. 4,978,517 and 5,174,868) and chloride (U.S. Pat. No. 5,458,858; Canadian Pat. Nos. 461586 and 782574). The disclosures of these listed patents are hereby incorporated by reference. These commercial processes incorporate the following chemical reactions:
6NaClO.sub.3 + 3H.sub.2 SO.sub.4 + 3H.sub.2 O.sub.2 .fwdarw. 6ClO.sub.2 + 6H.sub.2 O + 3Na.sub.2 SO.sub.4 + 3O.sub.2 6NaClO.sub.3 + 3H.sub.2 SO.sub.4 + 2CH.sub.3 OH .fwdarw. 6ClO.sub.2 + 5H.sub.2 O + 3Na.sub.2 SO.sub.4 + O.sub.4 + 2HCOOH NaClO.sub.3 + Alcohols + H.sub.2 SO.sub.4 .fwdarw. ClO.sub.2 + Na.sub.2 SO.sub.4 + CO.sub.2 + H.sub.2 O 6HClO.sub.3 + 3H.sub.2 O.sub.2 .fwdarw. 6ClO.sub.2 + 6H.sub.2 O + 3O.sub.2 6NaClO.sub.3 + 12HCl .fwdarw. 6ClO.sub.2 + 3Cl.sub.2 + 6H.sub.2 O + NaCl
There are many drawbacks involved with the use of these existing technologies. For instance, methanol is a poisonous and volatile chemical. In addition, as shown above, the major oxidized product formed in methanol reduction processes is formic acid, which is also a toxic chemical. Thus, the use of methanol as a reducing agent in the production of chlorine dioxide creates an environmental hazard.
Chloride as a reducing agent is supplied either as hydrogen chloride or as sodium chloride. While chloride is not expensive, its use as a reducing agent in these processes is also disadvantageous since it makes chlorine dioxide with large amounts of chlorine impurity. This is problematic since there has traditionally been no practical means of disposing of chlorine waste material.
The use of methanol as a reducing agent in chlorine dioxide processes also has the disadvantage of producing the inevitable by-product sodium sulfate (Na.sub.2 SO.sub.4), which is commercially known as salt cake. A part of the salt cake by-product is consumed by pulp mills to make sodium sulfide, a component of black liquor. However, the bulk of the salt cake does not have any use and must simply be disposed of.
Hydrogen peroxide can also be used as an efficient reducing agent in the production of chlorine dioxide. Although expensive, the use of hydrogen peroxide is an attractive alternative since it does not result in the formation of the formic acid and chlorine toxic by-products. However, the use of hydrogen peroxide also results in the production of large amounts of undesirable salt cake as a waste product.
There is therefore a need in the art for an improved method of producing chlorine dioxide.
It is therefore a primary objective of the present invention to provide a method of producing chlorine dioxide which does not form toxic by-products and is environmentally safe.
It is a further objective of the present invention to provide a means of producing chlorine dioxide in higher yield with a faster reaction rate than commercially known processes.
It is yet a further objective of the present invention to provide a means of producing chlorine dioxide which is economical.
It is a further objective of the present invention to provide a means of producing chlorine dioxide without the need for recycling the by-products.
These and other objectives will become apparent from the following description.